1. Field of the Invention
This invention relates to pipelines which are lined with a plastics material.
Pipelines are constructed by first forming lengths or “stalks” of pipe, and subsequently joining the stalks together. The stalks may be joined in the field for a land-based pipeline. for a subsea pipeline, the stalks may be joined on a lay barge, or may be joined while the pipe is being reeled onto a reel laying vessel.
In many applications a metal, typically a steel, pipeline must be lined with a plastic liner in order to prevent corrosion of the internal surface of the pipe because of the nature of the fluid to be transported and on conditions of use such as pressure and temperature. When connecting stalks of lined pipe together, it is necessary to ensure that the corrosion protection provided by the liner is continuous across the connection.
In onshore use, it is common to connect together stalks of pipe by flanged connections, with the liner material being trapped between the flanges. However, in some land based applications and in many subsea uses, it is not possible to use flanged connections. This is particularly the case when laying undersea pipelines by reel-laying methods.
In these cases it is necessary to connect the pipe stalks by butt welding, and to do so in such a way that the plastic liner is not degraded by the heat of welding. Various means of achieving this have been proposed in the prior art.
It is also known to use plastic liners which are provided on their outer faces with continuous longitudinal grooves. This is done because gas within the transported fluid can permeate through the liner and accumulate at the liner/pipe interface. The grooves allow the gas to travel along the interface to some point where a gas draw-off is provided through the pipe. Prior art methods of joining lined pipe stalks do not permit such grooves or equivalent gas seepage paths to extend across the join.
In the case of reeled pipeline for offshore applications, the assembly of the stalks is made during reeling of the pipe onto the vessel. It is therefore important to reduce the time taken for connecting the stalks, in order to minimise the immobilisation of the vessel.
2. Description of Related Art
U.S. Pat. No. 5,998,691 shows a method of connecting stalks of lined pipe which makes use of pup pieces welded within the ends of the pipe stalks. This method requires continuous corrosion-resistant welds to be made between the pipes and the pup pieces. This method is complicated to implement and takes time. Also, it cannot provide gas drainage grooves extending across the join.
U.S. Pat. No. 5,348,211 also relies on the use of metal inserts welded into the pipe stalk ends, with similar disadvantages.
EP 0,366,299 shows an arrangement using a thermoplastic insert which carries a ceramic ring on top of which welding is performed, insulation being provided between the ceramic ring and the thermoplastic material. However, the arrangement disclosed in this document has the disadvantage that the location of the ceramic ring can vary; this is because the liners and the insert are typically joined under tension, and when abutting the external pipe there can be no assurance that the liner has kept its position. Also, during service the liner can move within the pipe. It is therefore difficult to ensure that the ring is positioned under the pipe welding location in all circumstances. Also, this prior art considerably reduces the size of the internal bore while keeping the thickness of the liner at the connection, and it cannot be implemented with a grooved liner.